1. Field of the Invention
The present invention relates to a toner-carrying roller such as a developing roller and an image forming apparatus provided therewith which are used in an electrophotographic apparatus and electrostatic recording apparatus (such as copiers and printers) to supply a toner to an image-forming body (such as photoconductive drum, belt, paper, OHP, and photographic paper), thereby forming thereon a visible image. More particularly, the present invention relates to a toner-carrying roller and an image forming apparatus provided therewith that provide high-quality images free of density fluctuation and fogging and maintain their performance without deteriorating images for a long period of time.
2. Description of the Prior Art
In electrophotographic apparatus and electrostatic recording apparatus (such as copiers and printers), the photoconductive drum (or any other body) supporting a latent image thereon is supplied with a toner which adheres to the latent image, thereby making it visible. This developing process is known as the pressure developing process. (U.S. Pat. Nos. 3,152,012 and 3,731,146) This process offers the advantage of requiring no magnetic material, which leads to a simpler, smaller apparatus. This process also permits a color toner to be used easily.
The pressure developing process is designed to carry out development in such a manner that the toner-carrying roller carrying a toner (usually a non-magnetic one-component developing agent) is brought into contact with the photoconductive drum (or any other body) supporting an electrostatic latent image so that the toner adheres to the latent image on the photoconductive drum. Therefore, it is necessary that the toner-carrying roller be formed from an electrically conductive elastic material.
As illustrated in FIG. 2, the pressure developing process employs a toner application roller 4 to supply a toner, a photoconductive drum 5 to hold an electrostatic latent image, and a toner-carrying roller 1 placed between them. As they turn in their respective directions indicated by arrows, the toner application roller 4 supplies the toner 6 to the surface of the toner-carrying roller 1. The thus supplied toner 6 is spread thin uniformly by the spreading blade 7. The toner-carrying roller 1 (carrying the toner 6 thereon) and the photoconductive drum 5 turn while keeping contact between them, so that the toner 6 (in the form of thin layer) adheres to the latent image on the photoconductive drum 5, making the latent image visible. In FIG. 2, the reference numeral 8 represents the image transfer unit which transfers the toner image to the recording medium (such as paper), and the reference numeral 9 represents the cleaning unit which, by means of the cleaning blade 10, removes residual toner remaining on the surface of the photoconductive drum 5 after image transfer.
For the above-mentioned process, it is necessary that the toner-carrying roller 1 and the photoconductive drum 5 rotate while keeping a close contact between them. To this end, the toner-carrying roller 1 is composed of a shaft 2 of good conducting material (such as metal) and an elastic, electrically conductive layer 3 made of elastic rubber (such as silicone rubber, NBR, and EPDM) or urethane foam incorporated with a conductive agent, as shown in FIG. 1.
There is another method for forming an image as disclosed in Japanese Patent Laid-open No. 116559/1983. This method employs a latent image holder and a developing sleeve which are arranged in close proximity to each other (without contact). The developing sleeve carries a non-magnetic toner in the form of thin layer, which jumps onto the latent image holder. In its modifications, the latent image holder takes the form of a belt instead of a drum. Further, a recording medium such as a sheet of plain paper, OHP, and photographic paper may be adopted, whereby a toner is supplied directly to the recording medium to form a visible image on the recording medium. For example, as in a mechanism disclosed in Japanese Patent Laid-open No. 129293/1996, a back electrode roller is provided on the back side of a recording medium, and a toner-carrying roller carrying a toner is provided on the front side of the recording medium in the vicinity thereof. The toner on the toner-carrying roller is controlled by an aperture electrode and jumped toward the back electrode roller, thereby supplying the toner to the recording medium present between the back electrode roller and the toner-carrying roller to form a visible image on the recording medium. These processes may also employ the same toner-carrying roller as mentioned above.
The conventional toner-carrying roller mentioned above, however, suffers the disadvantage of being liable to cause fogging, particularly in the case where a non-magnetic toner is used. To eliminate this disadvantage, it is necessary to carry the toner in the form of uniform thin layer, while keeping it evenly highly charged. This necessitates an ability to adequately control the surface properties of the roller. One way to achieve this object is attained by making the surface of the toner-carrying roller as smooth and even as possible or by attaching to the surface of the conductive layer 3 of the toner-carrying roller a resin which contributes to the electrification of the toner. In this way it is possible to improve the surface smoothness and charging performance. Nevertheless, the resulting toner-carrying roller does not necessarily give satisfactory performance because it is impossible to detect subtle changes on the roller surface and hence it is impossible to quantitatively control the characteristic properties of the roller surface. Thus the toner-carrying roller usually causes a problem with deterioration in image quality after a long period of use. There even is an instance where the above-mentioned means does not prevent poor images (with fogging and decreased density) from occurring from the beginning of operation and on.